Psalms 40:9

9.I have proclaimed thy righteousness in the great assembly. Here David again brings forward his own thankfulness, and for no other reason but to induce God to continue his goodness towards him. God, whenever he manifests his liberality towards us, encourages us to render thanks to him; and he continues to act in a similar manner towards us when he sees that we are thankful and mindful of what he has done for us. In the first place, David makes use simply of the word righteousness; but it must be understood of the righteousness of God, which he expressly mentions soon after. Nor does he say, that it was only in the secret affection of the heart, or in private, that he offered praise to God, but that he had openly proclaimed it in the solemn assembly, even as the faithful in those days were wont to testify their devotion by presenting peace-offerings to God when they had been delivered from any great danger. The great assembly of which he speaks is not to be understood of the concourse of people that assemble at courts of law, or at the public market-places, but it denotes the true and lawfully constituted Church of God, which we know assembled in the place of his sanctuary. Accordingly, he declares that he had not concealed in his heart the righteousness of God, which it becomes us publicly to make known for the edification of one another. Those who keep it hid in their hearts are surely seeking as much as in them lies that the memory of God may be buried in oblivion. He calls upon God as a witness of this, not only to distinguish between himself and hypocrites, who often proclaim loudly, and with all their might, the praises of God, and yet do so without the least spark of affection; but also to make it the more abundantly obvious that he had sincerely and heartily uttered the praises of God, and was careful not to defraud him of any part of them. This affirmation teaches us that the subject which is here treated of is one of no small importance; for although God stands in no need of our praises, yet it is his will that this exercise for many reasons should prevail amongst us.